This study involves five groups. Participants will be randomly assigned to one of these groups. All groups will receive information on a dietary approach aimed at losing weight. All groups will be asked to favor foods that are low fat and have a low glycemic index, which means they will be asked to favor foods that don't cause a quick rise in blood sugar (for example, choosing oatmeal over cornflakes for breakfast). All groups will also be asked to take a vitamin B-12 supplement each day of their choosing (can be a multivitamin).